1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a ferroelectric capacitor that uses a bottom electrode mixture of a noble metal and noble metal oxide to promote c-axis nucleation and growth of a PGO ferroelectric film.
2. Description of the Related Art
Pb5Ge3O11 (PGO) single-phase thin films exhibit ferroelectric properties when oriented along their c-axis. A c-axis oriented PGO film has desirable memory characteristics that make it suitable for use in one-transistor (1T) memory devices.
It has been found that PGO thins film grown on an Ir substrate exhibit stronger c-axis orientation than PGO grown on a Pt substrate, under equivalent deposition conditions. By comparing the XRD spectrum of Ir and Pt substrates (see FIGS. 1A and 1B), it can be seen that the Pt substrate shows a stronger preference for (111) peaks than Ir, at comparable film thicknesses. Since the crystal structure and lattice constant of Ir and Pt are about the same, 3.839 and 3.923 respectively, and the lattice constant of PGO is a=10.251 and c=10.685, the question arises as the whether the crystalline orientation may be a factor in promoting c-axis PGO nucleation and growth.
Although Ir bottom electrodes are known to enhance nucleation and the growth of c-axis PGO, Ir has one significant drawback. Ir is difficult to polish. Therefore, it can be a problem to use simple chemical-mechanical polish (CMP) processes, to form self-aligned gate structures in one-transistor integration, when using Ir as bottom electrode. Typically, an etching process must be used to define the Ir bottom electrode, which can induce etching damage at the gate and source/drain regions. Furthermore, a buffer polysilicon layer and a conductive barrier layer between the bottom electrode and polysilicon gate are usually needed in order to reduce the etching damage to the source drain region. These extra steps make the 1T integration process more complicated. However, Pt is easily polished.
It would be advantageous if Pt bottom electrodes could be used in 1T fabrication processes instead of Ir, to simplify the polishing processes.
It would be advantageous if Pt bottom electrodes could be formed that promote c-axis orientation in an overlying PGO ferroelectric thin film.